An engine may be used to operate two or more separate drives. This presents difficulty when the drives are to be operated at variable rates with respect to each other.
One example of this situation is amphibious vehicles. Amphibious vehicles normally have one propeller at the front and one propeller at the rear. When such a vehicle travels on land, the propellers are halted and the engine of the vehicle drives the vehicle in the traditional manner via the wheels. In water, the propellers are connected via the transmission to the engine that drives the wheels.
Due to the fixed transmission ratios between the two propellers, the propellers rotate at a predetermined, fixed speed ratio. This impairs the maneuverability of the amphibious vehicle.